JP6908849B2 - Synchronous rectifier circuit and switching power supply - Google Patents

Description

The present invention relates to a synchronous rectifier circuit and a switching power supply device.

The switching power supply device is used as an AC (Alternating Current) / DC (Direct Current) converter or a DC / DC converter.
Conventionally, the primary side circuit unit (the circuit unit on the side that receives power from the AC power supply in the AC / DC converter) and the secondary side circuit unit (the circuit unit on the side that outputs the DC voltage in the AC / DC converter) are separated. There is a switching power supply that is electrically isolated and magnetically connected using a transformer. Further, as a circuit for rectifying the voltage waveform generated in the secondary winding of the transformer, a transistor connected to the secondary winding (hereinafter, may be referred to as a secondary side switch) is used, and the secondary winding is performed at a timing based on the voltage waveform. There is a synchronous rectifier circuit that rectifies by turning the side switch on or off. In recent years, a control circuit such as a dedicated control IC (Integrated Circuit) is often used in order to control the secondary side switch with high accuracy and further improve the conversion efficiency.

By the way, the operation mode of the switching power supply device includes a current discontinuous mode, a current critical mode, and a current continuous mode. In the current discontinuity mode, the switching transistor (hereinafter sometimes referred to as the primary side switch) included in the primary side circuit section and the current flowing through the secondary side switch are both simultaneously for a certain period in each cycle of the current waveform. This is an operation mode in which each switch is controlled so as to be 0A. The current critical mode is an operation mode in which each switch is controlled so that the current flowing through each of the switches becomes 0A at the same time at one point in each cycle of the current waveform. The current continuous mode is an operation mode in which each switch is controlled so that there is no period or timing during which the currents flowing through the switches are both 0A at the same time. In the continuous current mode, a larger output current can be obtained than in other modes.

However, in the current continuous mode, when the primary side switch is turned on while the current is flowing in the load direction through the secondary side switch, the secondary winding of the secondary side switch is caused by magnetic coupling by the transformer. The voltage of the terminal connected to may be a large positive value. At this time, if the secondary side switch remains in the ON state due to the operation delay of the secondary side switch, a large current flows to the reference potential side via the secondary side switch, and a large power loss occurs.

Conventionally, there has been a technique of intentionally delaying the operation of the primary side switch to prevent the primary side switch and the secondary side switch from being turned on at the same time.

Japanese Unexamined Patent Publication No. 9-285116 Japanese Unexamined Patent Publication No. 11-23502 Japanese Unexamined Patent Publication No. 2003-333846

In the conventional technique of intentionally delaying the operation of the primary side switch, a dead time occurs in which both the primary side switch and the secondary side switch are turned off. In order to realize the current continuous mode, a diode may be connected in parallel with the secondary side switch so that the current flows to the load side even in this dead time. When a Si (silicon) -MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is used as the secondary switch, the diode (body diode) structurally contained in the Si-MOSFET fulfills its function. ..

However, since the forward voltage of the diode is larger than the voltage across the secondary switch when it is turned on, there is a problem that the power loss becomes large when the dead time becomes long and the period in which the current flows through the diode becomes long.

In one aspect, it is an object of the present invention to provide a synchronous rectifier circuit that reduces the power loss of a switching power supply and a switching power supply.

In one embodiment, the first terminal connected to one end of the secondary winding of the transformer included in the synchronous rectification type switching power supply device, the second terminal serving as the reference potential, and the first control voltage are A first transistor having a third terminal to be applied and performing a switching operation based on the first control voltage, a cathode is connected to one end of the secondary winding, and an anode is connected to the reference potential. A diode, a first control circuit that outputs a second control voltage that controls the switching operation of the first transistor based on the first voltage of the first terminal, and a primary winding of the transformer. The first control voltage is set based on the third control voltage output by the second control circuit that controls the switching operation in the second transistor connected to one end of the wire and the second control voltage. The control voltage generation circuit to be generated and the on period during which the diode is turned on are detected based on the first voltage, and the longer the on period is, the more between the second control circuit and the second transistor. Provided is a synchronous rectifier circuit having a delay time control circuit for controlling the delay circuit provided to delay the third control voltage so that the delay time is shortened.

Also, in one embodiment, a switching power supply is provided.

On one side, the power loss of the switching power supply can be reduced.

It is a figure which shows an example of the switching power supply device and the synchronous rectifier circuit of 1st Embodiment. It is a figure which shows an example of the current waveform of three operation modes. It is a figure which shows an example of the operation waveform of the switching power supply device when the delay circuit and the control voltage generation circuit are used. It is a figure which shows an example of the switching power supply device of the 2nd Embodiment. It is a figure which shows an example of the corrugated form molding machine. It is a figure which shows another example of a corrugated molding machine. It is a figure which shows an example of the input waveform and the output waveform of a waveform molding machine. It is a timing chart which shows an example of the output signal of a voltage inverter, the reference voltage, and the output signal of a comparator. It is a timing chart which shows an example of the DC voltage output by the first rectifier circuit, the reference signal (triangular wave), the output signal of the comparator, and the DC voltage output by the second rectifier circuit. It is a figure which shows the example of shortening the on-period of a diode. It is a figure which shows an example of a flyback type AC / DC converter. It is a figure which shows an example of a DC / DC converter.

Hereinafter, embodiments for carrying out the invention will be described with reference to the drawings.
(First Embodiment)
FIG. 1 is a diagram showing an example of a switching power supply device and a synchronous rectifier circuit according to the first embodiment.

The switching power supply device 10 is used as an AC / DC converter or a DC / DC converter. Further, the switching power supply device 10 is used as, for example, a POL (Point Of Load) power supply, and can carry an output current of several tens to several hundreds of amperes.

The switching power supply device 10 includes a primary side control circuit (hereinafter referred to as a primary side control IC) 11, a delay circuit 12, and a transistor 13 included in the primary side circuit unit. In FIG. 1, the other elements of the primary circuit portion of the switching power supply device 10 are not shown. When the switching power supply device 10 is used as an AC / DC converter, a rectifying unit for rectifying an AC voltage and the like are included in the primary circuit unit.

Further, the switching power supply device 10 has a transformer 14 that electrically insulates the primary side circuit portion and the secondary side circuit portion and magnetically couples them. By having the transformer 14, the switching power supply device 10 can prevent the influence of an electrical short circuit from being transmitted to the other when an electrical short circuit occurs in one of the primary circuit unit or the secondary circuit unit. The secondary circuit unit includes a synchronous rectifier circuit 15.

In the following, it is assumed that the transistor 13 is an n-channel type FET (Field effect transistor). Examples of FETs include Si-MOSFETs, FETs using GaN (gallium nitride), and FETs using GaAs (gallium arsenide).

The primary side control IC 11 receives a power supply voltage (not shown) and outputs a control voltage Vg1a for controlling the switching operation of the transistor 13 at a predetermined frequency (hereinafter referred to as a switching frequency). For example, the primary side control IC 11 determines the ratio of the on-time of the transistor 13 in one cycle (hereinafter, duty ratio) depending on whether the switching power supply device 10 is operated in the above-mentioned current discontinuous mode, current critical mode, or current continuous mode. ) Is changed. The duty ratio can be changed by changing the pulse width of the control voltage Vg1a.

The primary side control IC 11 receives an error signal indicating an error between the output voltage (DC voltage) of the switching power supply device 10 and its expected value, and adjusts the duty ratio to an appropriate value based on the error signal. You may do so.

Further, the primary side control IC 11 is connected to a terminal (hereinafter referred to as GND) that serves as a reference potential (hereinafter referred to as 0V, but is not particularly limited to 0V).
The delay circuit 12 generates a gate voltage Vg1 that delays the control voltage Vg1a and supplies it to the gate terminal of the transistor 13. The delay time for delaying the control voltage Vg1a can be adjusted by the control signal ct output from the synchronous rectifier circuit 15.

The transistor 13 has a drain terminal connected to one end of the primary winding 14a of the transformer 14, a source terminal connected to the GND, and a gate terminal connected to the delay circuit 12. The transistor 13 performs a switching operation based on the gate voltage Vg1.

The transformer 14 has a primary winding 14a, a secondary winding 14b, and a core 14d. Although schematically shown in FIG. 1, the primary winding 14a and the secondary winding 14b are wound around the core 14d. The black circle shown near one end of each winding indicates the position of the winding start of each winding.

One end of the primary winding 14a is connected to the drain terminal of the transistor 13. The other end of the primary winding 14a is not shown, but when the switching power supply 10 is an AC / DC converter, it is connected to a rectifying unit that rectifies the AC voltage, and the switching power supply 10 is a DC / DC converter. In the case of, it is connected to a DC power supply. One end of the secondary winding 14b is connected to the output terminal OUT of the switching power supply device 10. The other end of the secondary winding 14b is connected to the synchronous rectifier circuit 15.

The synchronous rectifier circuit 15 includes a transistor 15a, a diode 15b, a secondary side control circuit (hereinafter referred to as a secondary side control IC) 15c, a control voltage generation circuit 15d, a delay time control circuit 15e, and a resistance element 15f. As with the transistor 13, in the following, the transistor 15a is assumed to be an n-channel type FET.

The transistor 15a has a drain terminal connected to one end of the secondary winding 14b, a source terminal connected to the GND and used as a reference potential, and a control voltage (hereinafter referred to as a gate voltage Vg2) supplied from the control voltage generation circuit 15d. It has a gate terminal to which it is applied. The transistor 15a performs a switching operation based on the gate voltage Vg2.

The diode 15b is provided so that the current flows to the load side even in the dead time when both the primary side switch and the secondary side switch are turned off. The cathode of the diode 15b is connected to one end of the secondary winding 14b, and the anode is connected to the GND to be the reference potential.

When the transistor 15a is a Si-MOSFET, the diode 15b may be a body diode formed in the Si-MOSFET.
The secondary control IC 15c is connected to the drain terminal of the transistor 15a via the resistance element 15f, and detects the drain voltage Vd2 that changes according to the change of the drain current Id2. Then, the secondary side control IC 15c outputs a control voltage Vg2a that controls the switching operation of the transistor 15a based on the drain voltage Vd2.

The secondary control IC 15c operates at the power supply voltage generated by rectifying the drain voltage Vd2. The circuit that generates the power supply voltage is not shown in FIG. Further, the secondary side control IC 15c is connected to the GND.

The control voltage generation circuit 15d generates a gate voltage Vg2 based on the control voltage Vg1a and the control voltage Vg2a.
The control voltage generation circuit 15d includes, for example, an inverting amplifier (inverter) 15d1, an AND (logical product) circuit 15d2, and an amplifier 15d3, as shown in FIG. Although not shown, each of these elements is connected to, for example, the output terminal OUT, and the output voltage at the output terminal OUT can be used as the power supply voltage.

A control voltage Vg1a is supplied to the input terminal of the inverting amplifier 15d1, and a voltage obtained by inverting the logical level of the control voltage Vg1a is output.
The AND circuit 15d2 calculates the logical product of the voltage obtained by inverting the logical level of the control voltage Vg1a and the control voltage Vg2a.

The amplifier 15d3 generates and outputs a gate voltage Vg2 by amplifying the output voltage of the AND circuit 15d2 to a value suitable for driving the transistor 15a.
The amplifier 15d3 may not be provided as long as the output voltage of the AND circuit 15d2 is a value suitable for driving the transistor 15a (this value differs depending on the type of the transistor 15a).

In such a control voltage generation circuit 15d, the output voltage of the AND circuit 15d2 becomes the L (Low) level at the timing when the control voltage Vg1a becomes the H (High) level. As a result, the gate voltage Vg2 also becomes the L level. Therefore, the transistor 15a is turned off regardless of the control voltage Vg2a.

The H level voltage is higher than the threshold voltage at which the transistors 13 and 15a are turned on, and the L level voltage is a voltage lower than the threshold voltage at which the transistors 13 and 15a are turned off (for example, 0 V). .. When the transistors 13 and 15a have different threshold voltages, the H level voltage and the L level voltage may be different in each of the transistors 13 and 15a.

The delay time control circuit 15e detects the on-period tdo in which the diode 15b is turned on based on the drain voltage Vd2, and controls the delay circuit 12 so that the longer the on-period tdo is, the shorter the delay time for delaying the control voltage Vg1a. do. As shown in FIG. 1, when the diode 15b is turned on, the drain voltage Vd2 becomes smaller by the forward voltage Vf of the diode 15b (the absolute value becomes larger). The delay time control circuit 15e detects the on-period tdo by detecting this change.

Hereinafter, before explaining the operation of the switching power supply device 10, the above-mentioned current discontinuous mode, current critical mode, and current continuous mode will be described.
FIG. 2 is a diagram showing an example of current waveforms of the three operation modes.

The waveform 20a is the waveform of the drain current Id1 of the transistor 13 of the primary side circuit unit in the current discontinuous mode, and the waveform 21a is the transistor 15a of the synchronous rectifier circuit 15 of the secondary side circuit unit in the current discontinuous mode. The waveform of the drain current Id2 of is shown. The waveform 20b is the waveform of the drain current Id1 of the transistor 13 in the current critical mode, and the waveform 21b is the waveform of the drain current Id2 of the transistor 15a in the current critical mode. The waveform 20c is the waveform of the drain current Id1 of the transistor 13 in the current continuous mode, and the waveform 21c is the waveform of the drain current Id2 of the transistor 15a in the current continuous mode.

However, in FIG. 2, the waveform 21c in the current continuous mode shows the waveform of the drain current Id2 when the delay circuit 12, the control voltage generation circuit 15d, and the delay time control circuit 15e shown in FIG. 1 are not used. There is. That is, the waveform of the drain current Id2 when the transistor 13 is directly driven by the control voltage Vg1a and the transistor 15a is directly driven by the control voltage Vg2a is shown. This is for comparison with the operation when the delay circuit 12, the control voltage generation circuit 15d, and the delay time control circuit 15e, which will be described later, are used.

In the following description, the drain current Id1 flowing in the direction from the drain terminal of the transistor 13 to the source terminal (terminal connected to the GND) is set as a positive value. On the other hand, the drain current Id2 flowing from the source terminal of the transistor 15a toward the drain terminal (flowing toward the output terminal OUT) is set as a positive value.

In the current discontinuous mode, there is a period ti0a in which the drain currents Id1 and Id2 of the transistors 13 and 15a both become 0A at the same time, and in the current critical mode, the timing ti0b in which the drain currents Id1 and Id2 of the transistors 13 and 15a both become 0A at the same time. There is. On the other hand, in the current continuous mode, there is no period during which the drain currents Id1 and Id2 of the transistors 13 and 15a both become 0A at the same time.

Further, as shown in FIG. 2, the periods tona, tomb, and tonc during which the transistor 15a is turned on in one cycle are the shortest in the current discontinuous mode and the longest in the current continuous mode.

When the transistor 15a is directly driven by the control voltage Vg2a, as shown in FIG. 2, a reverse current (current flowing from the drain terminal to the source terminal) may flow immediately after the period tonc. The reason is as follows.

When the gate voltage Vg1 of the transistor 13 rises from the L level to the H level, the transistor 13 is turned on, the drain current Id1 flows from the drain terminal to the source terminal, and magnetic energy is stored in the transformer 14. At this time, the drain voltage Vd1 is 0V. When the gate voltage Vg1 drops from the H level to the L level, the transistor 13 is turned off and the drain current Id1 becomes 0A. At this time, the drain voltage Vd1 rises from 0V.

When the transistor 13 is turned off, the diode 15b is first turned on by the magnetic energy stored in the transformer 14, and the diode current Id flows from the anode to the cathode. At this time, the drain voltage Vd2 changes to a negative value, and when the secondary side control IC 15c detects the change, the secondary side control IC 15c raises the control voltage Vg2a from the L level to the H level.

The secondary control IC 15c lowers the control voltage Vg2a from the H level to the L level when the drain voltage Vd2 rises as the drain current Id2 decreases and exceeds a certain threshold value. However, before the drain voltage Vd2 exceeds the threshold value (before the stored magnetic energy is completely extinguished), the drain voltage Vd1 drops to 0V by turning on the transistor 13. As a result, in the secondary circuit section, the drain voltage Vd2 rises to a large positive value even though the control voltage Vg2a is at the H level (despite the transistor 15a being in the ON state). Therefore, a large reverse current flows from the drain terminal of the transistor 15a toward the source terminal. That is, a large current is drawn from the load side (output terminal OUT side) to the transistor 15a side, and a large power loss occurs due to the high drain voltage and the large reverse current.

In order to reduce the power loss due to such a reverse current, a delay circuit 12 and a control voltage generation circuit 15d as shown in FIG. 1 are provided.
FIG. 3 is a diagram showing an example of an operation waveform of a switching power supply device when a delay circuit and a control voltage generation circuit are used. From the top, the waveforms of the control voltage Vg1a, the gate voltage Vg1 output by the delay circuit 12, the drain voltage Vd1, Vd2, the gate voltage Vg2, the drain current Id2, and the diode current Id are shown. In FIG. 3, the horizontal axis represents time, the vertical axis represents voltage in a graph showing a voltage waveform, and represents current in a graph showing a current waveform.

Note that FIG. 3 shows an operation waveform when the delay time control circuit 15e shown in FIG. 1 is not used. This is for comparison with the operation when the delay time control circuit 15e described later is used.

When the control voltage Vg1a rises from the L level to the H level (timing t1), the output voltage of the AND circuit 15d2 of the control voltage generation circuit 15d becomes the L level. As a result, the gate voltage Vg2 also becomes the L level. Therefore, the transistor 15a is turned off. Since the gate voltage Vg1 rises from the L level to the H level (timing t2) and the transistor 15a turns off before the transistor 13 turns on, the reverse current in the drain current Id2 of the transistor 15a shown by the waveform 22a Occurrence is suppressed.

However, from the timing t1 to the timing t2, a dead time occurs in which the transistors 13 and 15a are turned off at the same time, so that the diode current Id shown by the waveform 22b flows. As the diode current Id flows, the drain voltage Vd2 drops by the forward voltage Vf of the diode 15b.

The forward voltage Vf (absolute value is about 0.7V) of the diode 15b is larger than the drain voltage Vd2 (absolute value is about several milliV) of the transistor 15a in the on state. Therefore, when the dead time when both the transistors 13 and 15a are turned off becomes long and the period in which the diode current Id flows becomes long, the power loss becomes large.

The switching power supply device 10 of the first embodiment shortens the period during which such a diode current Id flows as follows.
As shown in FIG. 1, the delay time control circuit 15e detects the on-period tdo (the period from the timing t1 to the timing t2 in FIG. 3) of the diode 15b based on the drain voltage Vd2. As described above, the drain voltage Vd2 drops by the forward voltage Vf of the diode 15b due to the flow of the diode current Id, so that the delay time control circuit 15e detects the on-period tdo based on the drain voltage Vd2. can do. An example of how to detect the on-period tdo will be described later.

Then, the delay time control circuit 15e controls so that the longer the on-period tdo is, the shorter the time for the delay circuit 12 to delay the control voltage Vg1a. In the case of the delay circuit 12 in which the delay time becomes smaller as the voltage value of the control signal ct is larger, the delay time control circuit 15e outputs the control signal ct with a larger voltage value as the on-period tdo is longer.

As shown in FIG. 3, the shorter the delay time of the control voltage Vg1a, the shorter the on-period tdo. Therefore, the delay time control circuit 15e controls so that the longer the on-period tdo is, the shorter the delay time is, so that the on-period tdo of the diode 15b in one cycle of the switching operation of the transistor 15a is minimized. go. As a result, the power loss due to the diode current Id can be reduced.

Further, since the diode current Id flows during the dead time, the voltage rise between the drain terminal and the source terminal of the transistor 15a is suppressed, and the transistor 15a can be prevented from being destroyed.

When the switching power supply device 10 is used as a POL power supply, the transistor 15a becomes large and its parasitic capacitance also becomes large so as to be able to cope with a large output current of several tens to several hundreds of amperes. When the parasitic capacitance becomes large, the charging / discharging period of the charge to the parasitic capacitance during the switching operation becomes long, and if the timing for driving the transistor 13 is not properly controlled, a reverse current may be generated in the synchronous rectifier circuit 15. be. In the switching power supply device 10 of the first embodiment, the delay time control circuit 15e controls the delay time of the delay circuit 12 as described above, so that the dead time can be minimized while leaving a dead time. As a result, it is possible to prevent the generation of the reverse current and eliminate the power loss due to the reverse current, and it is also possible to reduce the power loss due to the diode current Id.

(Second Embodiment)
FIG. 4 is a diagram showing an example of a switching power supply device according to the second embodiment. In FIG. 4, the same elements as those shown in FIG. 1 are designated by the same reference numerals.

The switching power supply device 30 has a waveform shaper 31 which is an example of the delay circuit 12 shown in FIG.
The waveform shaper 31 deforms the voltage waveform of the control voltage Vg1a, which is a rectangular wave output by the primary side control IC11, to a degree based on the control signal ct, so that the rise time and the standing time with respect to the control voltage Vg1a The gate voltage Vg1 with a delayed fall time is output.

FIG. 5 is a diagram showing an example of a corrugated molding machine.
The corrugated shaper 31 includes a variable capacitance element 31a, a resistance element 31b, an inductor element 31c, a variable capacitance element 31d, and a resistance element 31e.

One end of the parallel circuit consisting of the variable capacitance element 31a and the resistance element 31b is connected to the primary side control IC 11, and the other end is connected to the gate terminal of the transistor 13 via the inductor element 31c.

One end of the parallel circuit consisting of the variable capacitance element 31d and the resistance element 31e is connected to the gate terminal of the transistor 13, and the other end is connected to the GND.
The variable capacitance elements 31a and 31d are, for example, varicap diodes, and change the capacitance value (capacitance value) based on the voltage value of the control signal ct. Hereinafter, the variable capacitance elements 31a and 31d will be described as having a smaller capacitance value as the voltage value of the control signal ct is larger.

In such a waveform shaper 31, as the voltage value of the control signal ct increases, the gate voltage Vg1 with a smaller rise time and a smaller delay of the rise time with respect to the control voltage Vg1a is generated.

FIG. 6 is a diagram showing another example of the corrugated molding machine.
The corrugated shaper 34 has a variable capacitance element 34a1 to 34an, a resistance element 34b1 to 34bn, an inductor element 34c1 to 34cn, a variable capacitance element 34d1 to 34dn, and a resistance element 34e1 to 34en.

The waveform molding device 34 is a circuit in which the same circuit as the waveform molding device 31 shown in FIG. 5 is connected in series between the n (n ≧ 2) stage, the primary side control IC 11 and the gate terminal of the transistor 13. Is.

By using such a waveform forming apparatus 34 instead of the waveform forming apparatus 31 shown in FIG. 5, the delay amount can be adjusted with a finer resolution than that of the waveform forming apparatus 31.
FIG. 7 is a diagram showing an example of an input waveform and an output waveform of the waveform molding device. In FIG. 7, the horizontal axis represents time and the vertical axis represents voltage.

The input waveform 35a indicates a time change of the control voltage Vg1a input to the waveform forming apparatus 31 or the waveform forming apparatus 34. The output waveforms 35b, 35c, 35d, and 35e indicate the time change of the gate voltage Vg1 output by the waveform forming apparatus 31 or the waveform forming apparatus 34 when the voltage value of the control signal ct is changed.

For example, the output waveform 35b shows the time change of the gate voltage Vg1 when the delay time is not controlled by the control signal ct in the waveform shapers 31 and 34. When the delay time is controlled by the control signal ct, as the voltage value of the control signal ct increases, the delay of the rise time decreases, for example, as in the output waveforms 35c, 35d, 35e.

Returning to the description of FIG.
In the synchronous rectifier circuit 32, the diode 32a, the resistance element 32b, and the capacitance element 32c generate a power supply voltage which is a DC voltage for operating the secondary side control IC 15c.

The anode of the diode 32a is connected to the secondary winding 14b, and the cathode is connected to one end of the resistance element 32b. The other end of the resistance element 32b is connected to one end of the capacitance element 32c and the power supply terminal of the secondary control IC 15c. The other end of the capacitive element 32c is connected to the GND.

The delay time control circuit 32d includes an attenuator 32d1, a voltage reversing device 32d2, a comparator 32d3, a rectifier circuit 32d4, a comparator 32d5, a reference signal generation source 32d6, a coupler 32d7, a rectifier circuit 32d8, and a voltage reversing device 32d9.

The attenuator 32d1 attenuates the drain voltage Vd2 to a value suitable for the input of the voltage reversing device 32d2. The attenuator 32d1 can be realized by using, for example, a resistance element.
The voltage inverter 32d2 inverts the polarity (positive or negative) of the voltage value of the output signal of the attenuator 32d1. The voltage inverting device 32d2 is, for example, an inverting amplifier.

The comparator 32d3 outputs a comparison result between the voltage value of the output signal of the voltage inverting device 32d2 supplied to the non-inverting input terminal and the reference voltage Vref applied to the inverting input terminal. The comparator 32d3 outputs an H level signal when the voltage value of the output signal of the voltage reversing device 32d2 is larger than the reference voltage Vref, and outputs an L level signal when the voltage value is smaller than the reference voltage Vref.

The reference voltage Vref is preset so as to be a value obtained by multiplying the product of the drain current Id2 when the transistor 15a is on and the on-resistance of the transistor 15a by the attenuation factor of the attenuator 32d1.

The rectifier circuit 32d4 rectifies the output signal of the comparator 32d3 and outputs a DC voltage. The rectifier circuit 32d4 has, for example, a diode 32d41, a capacitance element 32d42, and a resistance element 32d43, as shown in FIG. The output terminal of the comparator 32d3 is connected to the anode of the diode 32d41, and the inverting input terminal of the comparator 32d5 is connected to the cathode of the diode 32d41. One end of the capacitance element 32d42 and one end of the resistance element 32d43 are connected to the cathode of the diode 32d41, and the other end of the capacitance element 32d42 and the other end of the resistance element 32d43 are connected to the GND.

The comparator 32d5 outputs a comparison result between the DC voltage supplied to the inverting input terminal and the voltage value of the reference signal supplied to the non-inverting input terminal. The comparator 32d5 outputs an L level signal when the DC voltage is larger than the voltage value of the reference signal, and outputs an H level signal when the DC voltage is smaller than the voltage value of the reference signal.

The reference signal generation source 32d6 generates a signal whose voltage waveform is a triangular wave (for example, a sawtooth signal) as a reference signal. The reference signal generation source 32d6 may generate a reference signal having the same frequency as the frequency of the control voltage Vg1a output by the primary control IC11, or independently of the control voltage Vg1a. A reference signal having a frequency of about 1 to 10 times the frequency may be generated. The amplitude of the reference signal is set so that the duty ratio of the output signal of the comparator 32d5 becomes 0.5 when the DC voltage output by the rectifier circuit 32d4 is 0V, for example.

The coupler 32d7 transmits the output signal of the comparator 32d5 to the rectifier circuit 32d8. However, when the coupler 32d7 electrically insulates the output terminal of the comparator 32d5 and the input terminal of the rectifier circuit 32d8, an electrical short circuit occurs on either the primary side or the secondary side. Make sure that the effect is not transmitted to the other.

The coupler 32d7 is, for example, a transformer as shown in FIG. 4, and has windings 32d71, 32d72 and a core 32d73. One end of the winding 32d71 is connected to the output terminal of the comparator 32d5, and one end of the winding 32d72 is connected to the input terminal of the rectifier circuit 32d8. The other ends of the windings 32d71 and 32d72 are connected to the GND. Although schematically shown in FIG. 4, the windings 32d71 and 32d72 are wound around the core 32d73. The coupler 32d7 may be a photocoupler or the like.

The rectifier circuit 32d8 rectifies the output signal of the comparator 32d5 transmitted via the coupler 32d7 and outputs a DC voltage. The rectifier circuit 32d8 can be realized by the same circuit as the rectifier circuit 32d4, and has a diode 32d81, a capacitance element 32d82, and a resistance element 32d83.

The voltage reversing device 32d9 outputs a control signal ct with a smaller voltage value as the DC voltage output by the rectifier circuit 32d8 is larger. The voltage inverting device 32d9 is, for example, an inverting amplifier.

The delay time control circuit 15e of the synchronous rectifier circuit 15 shown in FIG. 1 can also be realized by the same circuit as the delay time control circuit 32d.
The capacitance element 33 of the switching power supply device 30 is provided to reduce the ripple voltage. One end of the capacitance element 33 is connected to the output terminal OUT, and the other end is connected to the GND.

Hereinafter, the operation of the delay time control circuit 32d in the synchronous rectifier circuit 32 of the switching power supply device 30 as described above will be described. Other operations are the same as the operations of the switching power supply device 10 of the first embodiment.

The drain voltage Vd2 is attenuated by the attenuator 32d1 and supplied to the voltage reverser 32d2 as a voltage proportional to the drain voltage Vd2. The output signal of the voltage inverter 32d2, the reference voltage Vref, and the output signal of the comparator 32d3 are, for example, as follows.

FIG. 8 is a timing chart showing an example of the output signal of the voltage reverser, the reference voltage, and the output signal of the comparator.
The voltage value of the output signal of the voltage reversing device 32d2 becomes larger than the reference voltage Vref in the on-period tdo when the diode 15b is turned on.

Therefore, the output signal of the comparator 32d3 becomes a pulse signal having an H level during the on-period tdo (for example, the period from the timing t10 to the timing t11 in FIG. 8). The pulse width of this pulse signal becomes wider as the on-period tdo is longer. That is, the pulse width reflects the on-period tdo.

The DC voltage output by the rectifier circuit 32d4 increases as the pulse width becomes longer (that is, the longer the on-period tdo is).
FIG. 9 is a timing chart showing an example of the DC voltage output by the first rectifier circuit, the reference signal (triangular wave), the output signal of the comparator, and the DC voltage output by the second rectifier circuit.

The voltage V1 indicates the DC voltage output by the rectifier circuit 32d4 when the on period tdo = tdo1, and the voltage V2 is the DC output by the rectifier circuit 32d4 when the on period tdo = tdo2 (<tdo1). It shows the voltage.

The smaller the DC voltage output by the rectifier circuit 32d4, the longer the period during which it becomes smaller than the reference signal. Therefore, the pulse width of the output signal of the comparator 32d5 becomes long.
For example, the pulse width pw1 when the DC voltage output by the rectifier circuit 32d4 is the voltage V2 is longer than the pulse width pw2 when the DC voltage is the voltage V1.

As shown in FIG. 9, the rectifier circuit 32d8 that receives the output signal of the comparator 32d5 via the coupler 32d7 outputs a smaller DC voltage as the pulse width of the output signal of the comparator 32d5 is shorter. For example, when the output signal of the comparator 32d5 is a pulse signal having a pulse width pw1, the rectifier circuit 32d8 outputs a voltage V2a as a DC voltage, and the output signal of the comparator 32d5 is a pulse signal having a pulse width pw2. In this case, the voltage V1a is output as the DC voltage.

The voltages V1 and V2 are generated based on the output signal (pulse signal) of the comparator 32d3, which is a frequency depending on the period in which the on-period of the diode 15b appears, and as shown in FIG. 9, it fluctuates slightly with time and an error occurs. ing. By increasing the frequency of the triangular wave, the frequency of the pulse signal output by the comparator 32d5 can be increased, and the accuracy of the voltages V1a and V2a output by the rectifier circuit 32d8 based on the pulse signal can be increased.

However, the DC voltage output by the rectifier circuit 32d8 becomes smaller as the on-period tdo of the diode 15b becomes longer. Is output so that the voltage is inverted.

As a result, the longer the on-period tdo, the larger the voltage value of the control signal ct is supplied to the waveform shaper 31. As described above, as the voltage value of the control signal ct becomes larger, for example, the capacitance values of the variable capacitance elements 31a and 31d shown in FIG. 5 become smaller, so that the delay time with respect to the control voltage Vg1a output by the primary control IC 11 becomes smaller. It gets shorter. When this delay time becomes short, the on-period tdo of the diode 15b becomes short.

In this way, by performing control so that the longer the on-period tdo is, the shorter the delay time is, the on-period tdo of the diode 15b in one cycle of the switching operation of the transistor 15a is minimized.

FIG. 10 is a diagram showing an example of shortening the on-period of the diode. FIG. 10 shows the current waveforms of the drain current Id2 and the diode current Id, and the voltage waveform of the gate voltage Vg2. Waveforms 36a and 37a indicate the drain current Id2 of the transistor 15a, and waveforms 36b and 37b indicate the diode current Id.

By shortening the on period of the diode 15b from the period tdoa to the period tdob, the period in which the diode current Id flows is shortened as shown in the waveform 37b. As a result, the power loss due to the diode current Id can be reduced, and the same effect as that of the switching power supply device 10 of the first embodiment can be obtained.

(Example of application to AC / DC converter)
The switching power supply device 30 as described above can be used as an AC / DC converter.

FIG. 11 is a diagram showing an example of a flyback type AC / DC converter. In FIG. 11, the same elements as those shown in FIG. 4 are designated by the same reference numerals.
The AC / DC converter 40 has a rectifying unit 41. The rectifying unit 41 rectifies the AC voltage and outputs a rectified signal. The rectifying unit 41 is, for example, a common mode choke filter connected to the AC power supply 41a via a fuse, a diode bridge that rectifies the AC voltage output by the common mode choke filter, and a capacitor that smoothes the rectified signal output by the diode bridge. Has. Further, the rectifying unit 41 may have a coil for blocking a high frequency signal included in the rectifying signal.

Further, the AC / DC converter 40 has a capacitance element 42 for supplying power to the primary side control IC 43. One end of the capacitive element 42 is connected to the output terminal of the rectifying unit 41, and the other end is connected to the GND.

The primary control IC 43 of the AC / DC converter 40 detects the voltage applied to the resistance element 44 connected between the source terminal of the transistor 13 and the GND, thereby detecting the current flowing through the source terminal of the transistor 13. Monitor. When the detected current is an abnormal value, the primary side control IC 43 stops, for example, the switching operation of the transistor 13.

Further, the voltage of the output terminal OUT is fed back to the primary control IC 43 via the photocoupler, and the output voltage of the AC / DC converter 40 is kept constant based on the fed back voltage. Adjust the duty ratio to an appropriate value. A bias stabilizing circuit 46 for stabilizing the bias is connected to the photocoupler 45.

Further, in the example of the AC / DC converter 40 of FIG. 11, the control voltage Vg1a output from the primary side control IC 43 is supplied to the control voltage generation circuit 48a of the synchronous rectifier circuit 48 via the coupler 47. The coupler 47 is, for example, a transformer similar to the coupler 32d7.

The control voltage generation circuit 48a has an attenuator 48a1 that attenuates the control voltage Vg1a supplied via the coupler 47 to a value suitable for the input of the AND circuit 15d2. Further, the control voltage generation circuit 48a attenuates the control voltage Vg2a output by the secondary control IC 15c to a value suitable for the input of the AND circuit 15d2. The attenuators 48a1 and 48a2 can be realized by using, for example, a resistance element.

The attenuators 48a1 and 48a2 may not be provided depending on the size of the control voltage Vg1a obtained via the coupler 47 and the size of the control voltage Vg2a.
In the AC / DC converter 40 as described above, the same effect as that of the switching power supply device 30 shown in FIG. 4 can be obtained.

(Example of application to DC / DC converter)
Further, the switching power supply device 30 as described above can also be used as a DC / DC converter.

FIG. 12 is a diagram showing an example of a DC / DC converter. In FIG. 12, the same elements as those shown in FIG. 11 are designated by the same reference numerals.
The DC / DC converter 50 is the same as the AC / DC converter 40 except that the rectifying unit 41 shown in FIG. 11 is not provided and the DC current source 50a is connected to one end of the capacitance element 42.

In the DC / DC converter 50 as shown in FIG. 12, the same effect as that of the switching power supply device 30 shown in FIG. 4 can be obtained.
Although one viewpoint of the synchronous rectifier circuit and the switching power supply device of the present invention has been described above based on the embodiment, these are merely examples and are not limited to the above description.

For example, although the transistors 13, 15a and the like in FIG. 1 have been described as being n-channel FETs, they may be p-channel FETs.

10 Switching power supply 11 Primary side control circuit (control IC)
12 Delay circuit 13,15a Transistor 14 Transformer 14a Primary winding 14b Secondary winding 14d Core 15 Synchronous rectifier circuit 15b Diode 15c Secondary side control circuit (control IC)
15d control voltage generation circuit 15d1 inverting amplifier 15d2 AND circuit 15d3 amplifier 15e delay time control circuit 15f resistance element ct control signal Id diode current Id1, Id2 drain current OUT output terminal tdo on period Vd1, Vd2 drain voltage Vf forward voltage Vg2 Gate voltage Vg1a, Vg2a Control voltage

Claims (5)

A first terminal connected to one end of the secondary winding of a transformer included in a synchronous rectification type switching power supply, a second terminal serving as a reference potential, and a third to which a first control voltage is applied. A first transistor having a terminal and performing a switching operation based on the first control voltage,
A diode in which the cathode is connected to one end of the secondary winding and the anode is the reference potential.
A first control circuit that outputs a second control voltage that controls the switching operation of the first transistor based on the first voltage of the first terminal.
Based on the third control voltage output by the second control circuit that controls the switching operation in the second transistor connected to one end of the primary winding of the transformer, and the second control voltage. A control voltage generation circuit that generates the first control voltage,
The on-period during which the diode is turned on is detected based on the first voltage, and the longer the on-period is, the more the delay circuit provided between the second control circuit and the second transistor is the third. A delay time control circuit that controls the delay time to delay the control voltage of
Synchronous rectifier circuit with. The delay time control circuit generates a first pulse signal in which the on period is reflected in the pulse width based on the first voltage, and rectifies the first pulse signal based on the pulse width. A first DC voltage of the same magnitude is generated, and a control signal for controlling the delay time is generated based on the first DC voltage.
The synchronous rectifier circuit according to claim 1. The delay time control circuit generates a third voltage by reversing the polarity of the second voltage proportional to the first voltage when the first transistor is on, and the first transistor generates a third voltage. The first pulse signal, which is the result of comparison between the reference voltage proportional to the product of the current flowing through the first transistor and the resistance of the first transistor when is on, and the third voltage. Generate,
The synchronous rectifier circuit according to claim 2. The delay time control circuit generates a second pulse signal based on the result of comparison between the first DC voltage and the reference signal which is a triangular wave, and is based on the result of rectifying the second pulse signal. Generate the control signal,
The synchronous rectifier circuit according to claim 2 or 3. A transformer containing a primary winding and a secondary winding,
The first transistor connected to one end of the primary winding and
A first control circuit that outputs a first control voltage that controls a switching operation in the first transistor, and a first control circuit.
A delay circuit that delays the first control voltage and supplies it to the first transistor.
It has a first terminal connected to one end of the secondary winding, a second terminal serving as a reference potential, and a third terminal to which a second control voltage is applied, and the second control A second transistor that performs switching operation based on voltage,
A diode in which the cathode is connected to one end of the secondary winding and the anode is the reference potential.
A second control circuit that outputs a third control voltage that controls the switching operation of the second transistor based on the first voltage of the first terminal.
A control voltage generation circuit that generates the second control voltage based on the first control voltage and the third control voltage.
Delay time control that detects the on-period during which the diode is turned on based on the first voltage, and controls so that the longer the on-period is, the shorter the delay time for the delay circuit to delay the first control voltage. Circuit and
Switching power supply with.

Images